1. Field of the Invention
This invention is in the field of thermal treatment of pulverulent material involving both a pre-calcining and a calcining zone, the pre-calcining being carried out in a highly turbulent gaseous atmosphere derived from the calcining zone, and including combustible fuel and preferably added oxygen containing gas, the gaseous atmosphere thus created passing through the pre-calcining zone at a very substantial linear velocity.
2. Description of the Prior Art
In the production of cement, argillaceous earth, lime, dolomite or the like, the heat treatment of the finely granular pulverized raw material is required. Typically, the prepared pulverized material is first delivered to a preheater section where it is suspended in a heat exchanger consisting of several cyclone separators connected in series and superimposed. Particles of the raw material are thereby preheated by the hot exhaust gases of a rotary kiln, the gases acting in countercurrent relationship to the pulverized material in the cyclone separators. Such thermal treatment provides deacidification of the pulverized material to a predetermined degree, before the particles are charged into the rotary kiln where they are calcined to completion. In this conventional method, almost the entire thermal energy is supplied to the rotary kiln, and a non-uniform distribution of heat occurs in the rotary kiln. The thermal degree of effectiveness and the output capacity of the furnace are limited and the life of the refractory bricks in the combustion zone is substantially reduced. In order to provide rotary kilns with a smaller cross-section and/or a furnace length, it has been attempted to undertake a low temperature calcination process of the pulverized raw material in a pre-combustion zone between the preheater and the furnace. For such a system to be effective, there must be an intimate mixture of fuel and raw material in the pre-combustion zone so that for the calcination of the pulverized raw material, the heat of the fuel may be imparted to the individual particles of the pulverized raw material.
In German Laid Open Specification No. 2,361,427 there is described a pre-combustion zone which is constructed in the nature of a shaft and contains a mixing zone laterally on the shaft for the separate feed of fuel and pulverized raw material used in the manufacture of cement. This mixture of fuel and raw material is then brought into contact with an upwardly directed stream of oxygen-containing gas received from a rotary kiln. In this type of arrangement, it is difficult to achieve a uniform mixture in the feed pipe and such a mixture can only take place with the aid of mechanical mixers. A further disadvantage of the mixing of fuel and pulverized raw material before they are directed into the stream of gas is that undesirable chemical reactions result between the fuel and the alkali-containing or sulfer-containing raw material which are detrimental to the entire calcining process.
In German Laid Open Specification No. 2,324,519 there is described another type of preheating arrangment utilizing a combustion zone between the preheater and the rotary kiln together with a fuel feed conduit in such a way that the fuel feed conduit is covered by the stream of material being introduced. In this arrangement, it is not possible to adjust the distribution of the quantities of material being introduced so that at predetermined points in the combustion zone, the concentration of raw material is so great that poor combustion or calcining conditions prevail. At other points, the concentration of raw material is relatively low so that at these points the particles reach especially high temperature peaks, with the consequence of over-calcination of the particles of material. The resulting coarsening of the grain and incipient melting appreciably impairs the flow capacity of the raw material and leads to disturbances in the entire installation.
German Laid Open Specification No. 2,365,653 describes an apparatus for the heat treatment of finely grained material with a preheating shaft in which exhaust gases from a rotary kiln flow. An additional combustion zone is provided through which the material is passed twice, once in a downward direction and upwards in the other. In order to achieve this result, the gas velocity must not be too great. Normally, the velocity of the combustion zone is not in excess of about 7 meters per second. This is typical for hot gas velocities currently used, in which the velocity is on the order of 7 to 20 meters per second. A uniform mixture of fuel and material in the preheating shaft and, therefore, a uniform heat treatment of the material is, however, not achieved as in the combustion zone the gases rising up cannot break up the clusters of material and the fuel gives off heat to the outer particles of the materials in the clusters, so that individual particles may be heated too high and others may be heated too little. In addition, such an installation provides undesirably high exhaust gas temperatures as the quantity of fuel theoretically adjusted to the quantity of the raw material cannot give off heat efficiently to the particles of pulverized raw material and the stream of hot gas is heated undesirably high.